资源环境科技发展态势分析平台

We analyzed the current stress state on the Nojima Fault, the source of the 1995 Mw 6.9 Kobe (Japan) earthquake, using breakouts in a borehole through the fault to a depth of similar to 1,000 m in 2017. The main fault was found at a depth of 529.3 m, with a damage zone similar to 60 m thick. Statistical analysis shows that the maximum horizontal stress (sHmax) in the depths shallower than 500 m rotates counterclockwise with depth toward the fault and reaches 138 degrees at a depth of 500 m approximately perpendicular to the fault, coinciding with that measured in 1997. In the depths of similar to 650-1,000 m, the sHmax is oriented 241 degrees, closely matching the regional tectonic stress direction. Our results reveal an accumulation of compressive stress, the driving force of faulting in the deep seismogenic zone since the 1995 earthquake, implying the fault zone has been healing continuously throughout the 22-year postseismic period.

Plain Language Summary Stress state, the condition of internal forces, around active fault is important because it can be a driving force for fault slip. We investigated the current stress state around the Nojima Fault, the source of the 1995 Kobe (Japan) earthquake, using the borehole breakouts. The borehole has been drilled through the fault to a depth of similar to 1,000 m in 2017, and the main fault plane was found at a depth of 529.3 m. Statistical analysis shows that the maximum horizontal stress (sHmax) in the deeper part above the fault in the borehole is oriented subperpendicular to the fault. On the other hand, sHmax below the fault is oriented closely matching the regional tectonic stress. Our findings reveal that the sHmax below similar to 650 m around the fault has recovered from realignment due to coseismic slip in the 22-year postseismic period, and now closely matches the regional tectonic stress direction.